Generally, valve opening and closing timings are set in order to enable an engine to obtain a maximum output at a specific rotation region (for example, specific revolutions per minute (RPM)). Therefore, in a low-speed rotation region, the valve opening and closing timings should be retarded for expansion and explosion of a mixture, while in a high-speed rotation region, the valve opening and closing timings should be advanced for discharge of the exploded mixture. However, when valve opening and closing timings are adjusted to a low speed, discharge of a mixture is retarded during a high-speed rotation, and, when the valve opening and closing timings are adjusted to a high speed, compression of the mixture is retarded during a low-speed rotation to cause significant degradation in efficiency of an engine.
To resolve such a problem, a continuously variable valve timing (CVVT) system is proposed in a manner that enables an engine to obtain high efficiency and high output at a high speed as well as a low speed by adjusting valve opening and closing timings in synchronization with the number of revolutions of the engine.
A CVVT mechanism is generally configured with a rotor vane mounted at one end part of a cam shaft, and a housing mounted at the rotor vane to form a retard angle chamber and an advance angle chamber inside the housing. Further, valve opening and closing timings are adjusted by controlling a pressure of oil that is supplied to the advance angle chamber and the retard angle chamber inside the housing by an oil control valve.
However, when foreign materials contained in the oil are stacked inside the oil control valve or in an oil flow path of the CVVT system, the CVVT system does not operate smoothly. To resolve such a problem, a related art discloses that a cleaning mode is performed to remove various kinds of foreign materials which are stacked between an oil flow path and a spool inside an oil control valve by moving the spool of the oil control valve a number of times from a full closed position (that is, a duty cycle of 0%) to a full open position (that is, a duty cycle of 100%), and vice versa by a control signal that is transmitted from an electronic control unit to the oil control valve.
Meanwhile, there occurs a case in which positions of an intake cam and an exhaust cam according to controlling of an oil control valve are varied due to causes such as extension of a timing chain that connects between a CVVT system and a cam shaft of an intake system, and the like. In this case, to compensate for a position of the cam shaft according to a control duty of the oil control valve, a controller performs learning of the CVVT system.
Typically, cleaning of the CVVT system and the above-described learning thereof are respectively performed in set operating regions. Further, when both the cleaning and the learning of the CVVT system are performed by a set number of times, they are not performed any longer.
However, when an operating region for the cleaning of the CVVT system is similar to that for the learning of the CVVT system and, as shown in FIG. 2, the cleaning and the learning of the CVVT system are performed at the same time, the learning has a priority higher than that of the cleaning so that a timing control of a continuously variable valve for the learning may be performed instead of the cleaning. Meanwhile, even though the timing control for the learning may be performed instead of the cleaning, the controller does not recognize such a situation to incorrectly recognize that a predetermined cleaning operation is performed based on a cleaning request signal.
Consequently, as shown in FIG. 2, even though the cleaning is not performed and the learning is actually performed, the number of times the cleaning is performed is counted to cause a reduction in an actual number of times the cleaning is performed. As a result, foreign materials are stacked inside the oil control valve to adversely affect durability of the oil control valve.